Mobility of lithium ions in anodic alumina formed on an Al–Li alloy
Introduction
Lithium is one of the important strengthening elements of commercial, precipitation-hardened, 8000 series aluminium alloys [1], which are normally subjected to surface treatment prior to service [2]. The pre-treatments commonly develop amorphous alumina films, sometimes hydrated, on the alloy surface; examples are acid pickling, alkaline etching, conversion coating and anodizing. From findings of previous work on aluminium alloys, it is anticipated that lithium atoms in solid solution in matrix regions of the alloy are incorporated immediately into the growing amorphous alumina films, presumably as Li+ ions, which migrate outward faster than Al3+ ions [3]. The immediate incorporation of lithium species is associated with the lower Gibbs free energy per equivalent for formation of Li2O compared with that for formation of Al2O3, while the faster migration of Li+ ions relates to the lower energy of the Li+–O bond compared with that of the Al3+–O bond [4]. Thus, lithium is not expected to enrich in the alloy as a consequence of film growth. Further, the Li/Al atomic ratio in the alumina film material should be less than that in the alloy by a factor dependent on the relative mobilities of Li+ and Al3+ ions. Due to the faster migration of Li+ ions, there is a possibility of formation of an outer film layer composed initially of Li2O or LiOH, depending upon the composition and pH of the environment [3]. If the efficiency of film growth is reduced, with lithium and possibly also aluminium species entering the solution, precipitated material may deposit at the film surface.
In the present work, the formation of a barrier anodic film, which is a model system for amorphous oxide films generally, is examined for a solid solution Al-3 at% Li alloy. The complementary techniques of elastic recoil detection analysis (ERDA), providing quantification of composition [5], and glow discharge optical emission spectroscopy (GDOES), providing enhanced depth resolution and sensitivity to lithium [6], are employed to determine the amount and distribution of lithium species in the film.
Section snippets
Specimen preparation
Specimens of Al-3 at% Li alloy, of approximately 6 cm2 working area, in the solution-treated condition (853 K for 60 min, followed by quenching to 273 K) were electropolished at 20 V for 300 s in perchloric acid/ethanol (20/80 by volume) electrolyte at 278 K. The specimens were then anodized at 5 mA cm−2 to 150 V in 0.1 M aqueous ammonium pentaborate electrolyte (pH 8.2) at 292 K. The voltage–time responses were recorded during anodizing. After both electropolishing and anodizing treatments,
Anodizing behaviour
The voltage–time response during anodizing of the alloy was linear, with slope V s−1 compared with for anodizing high purity aluminium, indicating film growth on the alloy at relatively high efficiency.
GDOES analysis
The GDOES composition profile for a specimen of the anodized Al-3 at% Li specimen reveals the typical, slowly rising signal from aluminium in the anodic film [6], prior to a rapid increase for the alloy substrate (see Fig. 1). Lithium species are present throughout the anodic
Discussion
Anodizing of aluminium under the present conditions is known to occur at an efficiency approaching 100%, with film growth proceeding at both the film/electrolyte and metal/film interfaces due to co-operative transport of Al3+ and O2− ions, respectively [9]. Boron species, which are immobile in the film, are found throughout the thickness of material formed at the film/electrolyte interface; this outer layer constitutes about 40% of the film thickness [10]. The small amount of lithium species in
Conclusions
- 1.
Anodizing of Al-3 at% Li alloy, in the solution-treated condition, at 5 mA cm−2 in ammonium pentaborate electrolyte at 292 K results in formation of an anodic alumina film contaminated uniformly by lithium species.
- 2.
The lithium species incorporated into the anodic film migrate outward about eight times faster than Al3+ ions, which is associated with a reduced energy of the Li+–O bond compared with that of the Al3+–O bond.
- 3.
The film forms at relatively high efficiency, with incorporation of immobile
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